Orthotrichum lyellii is the hero of this story. You’ve probably met it before: a tuft of green with soft tassels of leaves, stuck to a tree trunk or possibly a rock. Unlike plants with roots, this moss absorbs all it needs from air—an adaptation that allowed it to pinpoint poisons in Portland, Oregon, two years back.

In the spring of 2015, Oregon’s Department of Environmental Quality (DEQ) conducted tests as part of its Portland Air Toxics Solutions project. The department was monitoring dirty compounds that persist in the air via car exhaust, fuel burning, construction, and industrial processes.

The results were alarming. Concentrations of metals like cadmium and arsenic were three to six times higher than what is deemed safe. Elevated levels of this kind of toxin in the air can cause serious human health issues, including cancer and kidney, bone, and lung disease. Exposure to the metals can also impact neurological development in children. “We didn’t know the source, and we got pretty concerned,” says Oregon DEQ’s air toxics specialist Sarah Armitage.

DEQ was primarily interested in finding the source of the cadmium. The Portland office of the U.S. Forest Service, which had been testing air pollution for the previous two years and was interested in partnering with DEQ, guided the department to nature’s well-suited detective, Orthotrichum lyellii. Absorbent and rootless, moss and lichen are regularly employed as indicators of air pollution in European cities, explains the Forest Service’s Sarah Jovan. But environmental scientists in the United States are just now taking advantage of the valuable service that they offer.

Obtaining samples was a cinch. Orthotrichum lyellii grows particularly well on hardwood trees like maples and oaks. “We just went out and pulled a lot of moss off the tree trunks,” Jovan says.

After testing nearly 350 Orthotrichum lyellii samples from Portland’s trees, Jovan’s team crafted a moss-cadmium map for the city. It revealed two very toxic hot spots next to the Bullseye and Uroboros glass factories in different parts of town. Jovan and her colleagues shared their findings with Oregon’s DEQ that May.

What happened next was alarming. Five months passed before the DEQ placed air monitors near the metal hot spots that the moss testing had revealed. And the agency waited another three months before telling Bullseye that its southeast Portland factory was the prime source of the toxic cadmium emissions. When the public and Governor Kate Brown learned of the regulators’ lackadaisical approach to addressing such a serious case of air pollution, they cried foul.

A protest sicker on a street sign near the Bullseye Glass factory

Alex Milan Tracy/Sipa USA/Newscom

In February 2016, state senators called the findings a public health emergency. During a heated community meeting that month, the Portland Mercury reporter who broke the story asked state air quality manager David Munro why the DEQ allowed businesses to freely vent carcinogens like cadmium into the city’s air. “It’s a good question,” Munro responded. Oregon did not have sturdy laws or regulations in place to stop them from doing so. In March, the New York Timesreported locals weeping at public meetings and “raging” at their state officials. Home gardeners became afraid of their soils. Citizen air quality groups formed. Both Munro and Dick Pedersen, Oregon’s DEQ director, resigned.

Gradually, the glass companies and the government responded to the health scare. Both factories voluntarily stopped using arsenic and cadmium shortly after being notified about the moss study results. (The smaller, artist-owned factory, Uroboros, has since closed its Portland base.) After the compounds were put out of use, outdoor metal levels, as one would suspect, began to fall. In April 2016, DEQ established a set of glass-manufacturing rules that called for Bullseye to monitor its emissions. Today, the factory has returned to using arsenic (which rids the glass solution of bubbles) and cadmium (a key coloring ingredient in red and orange glass) but has installed new exhaust filters on its furnaces to keep the two toxins out of the atmosphere.

According to the DEQ, metal levels outside the Bullseye plant are now in sync with the background levels found throughout Portland. (The Cornell Waste Management Institute notes that there is no single standard that defines acceptable levels of these contaminants, however.) And the DEQ concedes that air monitors won’t always be spot on; windy weather and technology errors can get in the way of accurate readings.

Portland is now in the process of correcting its previously lax air regulations. Bullseye still does not have controls for other metals such as chromium or cobalt, which is used for blue glass. The company also had a serious, unaddressed lead issue. Moreover, a follow-up moss study found that nickel levels near one Portland industrial and metal factory were four times above health benchmarks. After this, DEQ quickly installed air-quality controls.

Beyond factories, car emissions like nitrogen and diesel are also a big concern in Oregon. When California tightened its own emissions rules, its northern neighbor became a dumping ground for hundreds of thousands of old trucks shooed off Golden State roads.

Portland, Oregon

Jamie Francis-Blue Chalk Media/Travel Portland

As a state that prides itself on its progressive environmental ethos, Oregon has grown especially sensitive to the issue of air pollution. In April of last year, the EPA ordered the state’s environmental health agencies to monitor for toxins via the Cleaner Air Oregon initiative. Now Oregon must comply with federal air-quality standards that it had been violating. But since these regulations aren’t tailored to specific facilities, emissions from individual businesses may still exceed lawful levels. Starting next fall, facilities statewide will be required to submit a detailed inventory of 600-plus potential chemicals they use, to help inform pollution prevention activities and related legislation. While about half of U.S. states, including California, have already gone above and beyond EPA regulations and adopted their own industry air-toxics control programs, Oregon has yet to do the same. Clearly, says Armitage, “we had an air-quality regulatory gap.” Regulators are relying on Cleaner Air Oregon to help fill that gap—the initiative’s timeline calls for new, permanent air-quality rules to be issued by the end of this year.

While public outcry helped to raise Portland’s awareness of its air problems, let’s not forget the original canary in the coal mine: the tree moss by the factories. Thanks to its absorbent powers, this easily overlooked plant revealed invisible toxins hiding in plain sight and spurred an effective movement to protect the city in whose midst it lived.